[DOC] Add link to "double backward" from "extending pytorch" page (#72584)
Summary:
It is probably the most user friendly to link to that (lesser known?) feature.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/72584
Reviewed By: soulitzer
Differential Revision: D34173999
Pulled By: albanD
fbshipit-source-id: 99fff7a55412faf54888f8317ab2388f4d7d30e4
(cherry picked from commit 2191ee76570b8c22a16aca75f947ece38e6ca3cf)
